From EP patent application 1 731 891, a method and apparatus for obtaining rotary angle related data of vehicle wheels in a wheel balancer is known. In that wheel balancer, vibration forces caused by imbalance of the wheel and variations of forces caused by a load roller bearing against the tyre are determined with respect to the rotary angle of the wheel. The determined rotary angle related vibration force data is used to calculate sizes and mounting positions of balancing weights for balancing the wheel. The determined rotary angle related load force data is used to improve balancing operation.
Another method or apparatus determining rotary angle related data of a vehicle wheel is known from EP patent 1 398 610 and relates to a vehicle wheel service apparatus such as wheel balancers or tyre mounting or demounting machines which are scanning the surface of the wheel by a preferably contactless sensor device in order to obtain geometrical data of the wheel. This geometrical data is rotary angle related data and may represent a three dimensional model of the geometrical shape and position of the wheel or of parts of the wheel. That rotary angle related data is used for diagnostic purposes or serves to improve the balancing operation or the tyre mounting operation in order to improve running characteristics of the wheel.
In the methods and apparatuses of the prior art, a rotary angle sensor is used in order to generated rotary angle position signals. That rotary angle position signals captured during measurement are then associated to the measurement data obtained from the force sensors or surface sensors in order to generate rotary angle related data such as imbalance measurement data, load force variation data or three dimensional geometrical data of the wheel. Such rotary angle sensors are required to detect the rotary position of the rotating wheel with a resolution sufficient to create suitable and precise rotary angle related data. Common rotary angle sensors comprise magnetic or optical sensors detecting the dents of a toothed wheel, for example, each tooth representing a certain rotary angle increment. Providing a rotary angle sensor having a suitable resolution increases both costs and number of parts of an apparatus.